Enzymatic Degradation of Heme. Oxygenative Cleavage Requiring Cytochrome P-450

A microsomal enzyme system has been described that converts heme (ferriprotoporphyrin IX) to the linear tetrapyrrole biliverdin IXα. On the basis of its absolute requirement for NADPH and molecular oxygen, its sensitivity to inhibition by CO, and its appropriate stoichiometry, the enzyme system tentatively was classified as microsomal heme oxygenase. It now has been demonstrated that bilirubin formed enzymatically from heme in the presence of molecular 18O2 contains 2 atoms of 18O and an additional 18O atom appears in the CO that originates from the α-methene bridge carbon of the heme. No 18O is incorporated into the bile pigment when the enzymatic reaction is carried out in a medium containing H218O instead of the molecular 18O2. In an atmosphere containing CO and O2 in a ratio of 1:2, the enzyme reaction is inhibited by 47%. Monochromatic light in the 450- to 470-nm range effectively reverses this inhibition. The photochemical action spectrum for this reversal closely resembles the absorption spectrum of the CO derivative of reduced cytochrome P-450. These findings indicate that the enzymatic conversion of heme to bilirubin is a mixed-function oxidation and that the microsomal enzyme system includes a hemoprotein with the characteristics of cytochrome P-450 as a terminal oxidase. The reaction does not appear to involve formation of intermediates such as verdoheme, which would require hydrolysis for conversion to biliverdin. © 1972, American Chemical Society. All rights reserved.